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The Office Worker's Portable Solar Oven

About: I like to tinker and I like to learn, and if one can support the other then thats great.
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In keeping with the grand tradition of creating ways to warm my lunch at work with readily available office supplies, I present for your review, dear reader The Office Worker's Portable Solar Oven.

Step 1: The Situation

Due to circumstances beyond my control, my cubicle at work is close to both the break room and a small loading area on the second floor. Because of this I usually have a wall of boxes filled with copier paper situated somewhere behind me. Aside from this being a nuisance when it comes to concentrating on my job it also affords me the opportunity to scavenge as many copier boxes that a man can get his hands on.

Step 2: The Result

Behold! My dashboard cooking days are over. I present to you The Office Worker’s Solar Oven. This project was built completely with materials laying about the office, with the exception of tinfoil, which I picked up at the dollar store.

Step 3: Gathering the Pieces

Above you can see all the precut pieces to form the main cooking chamber. I cut the box at an angle of 20 degrees to allow for maximum exposure to the sun. In my latitude the lowest elevation that the sun gets is 26 degrees above the Southern horizon.

Be sure to cut your box accordingly if you plan to make one.

Step 4: A Little Design Consideration

I designed the oven to allow for one inch of insulation on each side of the box. To do this I placed slots in the inside liner. This is the back piece.

Step 5: More Design Consideration

These are the sidepieces, slotted as well. I also cut these with an angle of 20 degrees.

Step 6: Some Assembly Required

As you can see the slotted pieces fit together nicely.

Likewise the liner fits nicely into the oven box.

Step 7: The Insulated Floor

Once I was sure that everything fit together, I removed all the pieces and put a one-inch spacer in the bottom of the box to keep the bottom of the oven above a layer of insulation.

The insulation I used was just shredded documents I salvaged from the office shredder.

Step 8: The Reflective Surfaces

Once properly stuffed I rapped the bottom liner of the oven in tinfoil and installed it into the oven.

I then wrapped each of the side liners in tin foil and assembled them.

Everything fits together perfectly.

Step 9: More Insulation

A little more insulation around the sides.

Don't forget all the nooks and crannies.

Step 10: The Window

For the port that allows the sunlight in I basically just created an insert that would fit into a regular box lid with a hole cut into it. I used the same trash bags they use to hold all the shredded documents and stapled it to a piece of cut out box.

This when gets inserted into the box lid with a hole cut about the same size in it.

Viola! The Office Worker's Portable Solar Oven

Step 11: A Question of Efficiency, Oh and a Little Math To.

This is all fine and dandy but what kind of power and efficiency are we talking about here. I mean can we bake bread, pasta, and stew?

I figured that for the first “light” of this oven it might be a good idea to run a test before I place a bunch of food in it and end up with a big mess and a half a dozen people laughing at my ruined lunch.

This will require some thought.

I will need to know a few things:
• How much potential wattage I can get out of a given area of sunlight
• How much wattage I AM getting out of a given area of sunlight
• How efficient is the set up based on these two values

To get potential wattage is easy.

The dimensions of the port that allows sunlight in are 40.64cm by 21.59cm. This equates to 877.4176 sq cm.

Each square meter of sunlight has a potential of 1000 watts of energy in it when it reaches the Earth. Each sq meter is 10,000 sq cm. So 1000 watts divided by 10,000 sq cm gives up .1 watts per sq cm.

So if we factor in the viewing area of the solar oven 877.4176 sq cm multiplied by .1 watts we get 87.74176 potential watts for the solar oven.

Potential wattage I have come to find out is often a pipe dream left for those that believe in endless amounts of power that can be conjured through a philosopher's stone at the stroke of midnight when moons are properly aligned. So I don’t readily buy into the thought of my copier paper box being able to harness 87 watts of power by merely being pointed at the sun.

I needed a way to measure the true wattage of this oven to be able to determine cooking times and more importantly what I could cook.

The easiest way to achieve this is to measure calories. This is done by multiplying the temperature change in Celsius by the mass of pure water heated in grams.

So I need to heat some water and measure the temperature change. However temperature change does not happen instantaneously, it takes time so that will need to be factored in down the road at some point.

Each calorie is equal to one degree centigrade increase in one gram of water. So with a given amount of water and the temperature change in the oven over a given amount of time, it should be enough to calculate its power.

Step 12: The Test Subject

For this I needed a small enough vessel for water. The container for the V8 I drink everyday will work fine. However I will need to paint it black to absorb the maximum amount of heat within the oven.

Here we see my test setup. The can has been painted black and filled with 163ml of filtered water. I plugged up the opening with a cork and stuck the probe of an oven thermometer in the center of this.

Step 13: The Test

And finally we see the entire test setup sitting out in the sun. The test consisted of leaving the entire apparatus in the sun for one hour and recording the temperature every moment.

This actually caused a small panic at my place of employment. Apparently the warehouse manager saw what looked to him as a strange device sitting out by one of the vehicles and it aroused a bit of concern. Lucky for me I managed to get a hold of him before anyone was called.

Step 14: The Numbers and of Course Math

This is a graph of the progress. The base temperature of the water was 77 degrees F. The time was 12:01 EST on 05/29/2008. The maximum elevation of the sun for that day was 71.8 degrees above the horizon. As can be noted there seemed to be a linear progression in regard to temperature increase. As I sat and observed I saw that the temperature went up one degree every two minutes. By the end of the hour the water was at 113 degrees. I would assume that it would have been higher if it were not for some clouds in front of the sun at the end of the hour.

To calculate the power of the oven I first needed to get the temperature change in Celsius.

77 F = 25 C113 F = 45 CTemp change = 20 C

163ml of water = 163 g

20deg * 163g = 3,260 Calories

Next we need to convert calories to joules. 1 Calorie = 4.1868 Joules.

3,260 Cal * 4.1868 = 13648.968 Joules

Now that we have the power in Joules we need to factor in the time to get wattage. Se we divide 13,648.968 Joules / 3,600 seconds (1 Hour)

3.971 Watts. Not very much. Hell even an Easy Bake oven uses a 100-Watt light bulb. But in the end one must remember that the heat is cumulative. The better the insulation the more heat will acquire and the better things will cook. It’s sad to report that out of 87 potential watts from the sun for the given area only a little fewer than 4 watts was produced. It would appear the overall the oven was only about 4.5% efficient

If nothing else this taught me a valuable lesson in the design considerations and power calculations necessary to build a more robust unit.

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53 Discussions

with regard to the reflective aspects of alu foil, that is a much 'heated' discussion. Apparently it is abt 50% and with that it is less than a white surface. Perhaps it would have been better to just line yr oven with white paper. That would have saved a run to the dollar store. :-)

You are correct, and in reality the list I can now form that would improve this project is far too long for me to even consider at this point seeing as how I have moved on to other things, namely http://zerosumhacker.blogspot.com/ :D

Fantastic instructable! I wonder if you would get more heat by painting the interior black to absorb light and heat to the box would work better than using foil. You still see people (thankfully not in my neighborhood) using foil on home windows to reflect light and heat away. You could use an outside hood (outside the main box) covered in foil to direct the light in (like a satalitte dish focusing on a smaller point) and use black paint on the inner box to absorb the heat.

I think it all depends on where you want the heat to go and what you want it to heat. It was my intention to bounce the light around inside the box and have it ultimately hit a black target that would absorb the heat.

In retrospect I see several areas for improvement of the model I presented here. But in reality I wanted to limit myself to what I have available around the office and of course I wanted to have fun.

Fantastic Instructable! Thanks for all the pics, the well explained directions, the quantification, and most importantly, for your time.

I would like to echo what Haw1horne said: Heating plastic is a bad idea unless it is designed for heating. You may not see or taste the outgassed products, but they are there. Other sites also say that if you can't use glass then the baking bags will work. I think a layer on the inside of the cover sealed with a cardboard edge, and another layer on the outside of the cover also sealed. Of course, pull tight to reduce diffraction.

An aside to the previous; careful what paint you use. Some are not safe to be heated. Look for those labeled as non-toxic stove paint.

A glass top would help keep the lid sealed because of its additional weight. If you go with the turkey bag window route, I'm thinking paperclip and rubber band anchoring system around the outside would keep things tight and allow for less heat loss. My guess is this is where your heat loss came from.

An angled lid lined with foil (aluminum, tin, lead, mylar, whatever) would catch more light and protect the works when all finished and folded down.

I'll start my oven tomorrow. Again, fantastic job with this and thank you so much for your time and effort.

if the insulation is to keep the heat in, i would suggest the paper shreds AND, a layer of mylar emergency blanket cut accordingly to fit the walls. because it reflects heat it is a good material to use for this, you can get them for like 7bucks online.

you, sir, are EXTREMELY good at making instructables. you were able to show how to make an oven (although you didnt really provide enough detail, but im sure the pictures are good enough for someone to tell what to do) andyou even did the math for us to show how hot it gets and all the important things the oven does. i salute you =]]

True! haha - although for information's sake (in case there was ever any kind of specific need for *tin* foil as opposed to aluminum), they actually still make tin foil these days - you can find it at florists shops that carry it. It's often preferred over aluminum at such places for some reason or other.